This invention relates to a monitoring device for electrically conductive resistive devices such as glow plugs used for starting diesel engines and has particular reference to the detection of failure of one of a number of glow plugs in such an engine.
Diesel engines though more expensive to manufacture than the normal Otto-cycle gasoline engine are increasing in both popularity and importance at the present, particularly for automotive devices. They can produce very good efficiency in terms of work done, or distance moved by a vehicle, for a given quantity of fuel consumed, compared with the gasoline engine.
In the past, the starting of such engines has been its major drawback. The compression ratio of diesel engines is very high, the temperature for ignition of the fuel injected into the cylinders being obtained solely by the compression of the air in the cylinders. Compression ratios are thus of the order of 24 to 1 and, therefore, large batteries are required for engine cranking. Alternatively, a supply of compressed air must be maintained, or generated independently such as by an auxiliary engine, for starting the engine. In the past, the starting of such engines, particularly in large trucks, has been so difficult that it has often been necessary to leave engines running for long periods, sometimes overnight, to avoid delays in getting underway subsequently.
This starting problem has largely been met in recent years by the use of glow plugs, which comprise essentially electrical wires suitably placed in the cylinders and made incandescent by battery current. A diesel engine so equipped can be started relatively easily upon cranking, injecting of fuel and supply of current to the plugs. After starting, the current supply is switched off.
One problem which arises in using such glow plugs is that, occasionally one may become defective or fail, so that ignition in the cylinder concerned is haphazard, and proper firing occurs only after the engine has been brought up to speed and warmed somewhat by the remaining operating cylinders. Since a typical engine will have 8 cylinders, one has the possibility of any one of 8 plugs failing at any time and the operator may not readily realize that a plug has failed, merely that the engine is a bit hard to start. With the general public now having diesel automobiles, it is even more important that a positive indication of a failed plug be given which otherwise might lead to a period of months before such defect would be found in periodic inspection such as when the vehicle is undergoing major servicing. Long periods when one of the cylinders is not starting properly is detrimental and can lead to engine damage, by washing of the cylinder walls by unburnt fuel and consequent wear. A reliable quick means of identifying a defective plug condition will thus save a great deal of time and expense in repair servicing, and also allow consistently better performance.
The present disclosure meets the problems outlined above and provides a system for the detection of defective glow plugs in a reliable and accurate manner using a circuit which can be set up quite simply on the engine or automotive vehicle concerned.